Acetic acid and acetic anhydride have long been used as basic chemicals for industrial purposes such as solvents, raw materials and intermeidates for various reaction products.
Acetic acid has been traditionally produced by oxidation of ethanol or acetaldehyde prepared by Wacker process starting from ethylene or low boiling raffinates produced in a petroleum process. These oxidation processes are being replaced by a carbonylation process which reacts methanol with carbon monoxide employing a rhodium catalyst in the presence of methyl iodide in liquid phase. However, the liquid phase carbonylation process has been found to have various critical deficiencies including the continuous loss of the expensive catalyst and the high corrosion problem stemming from the liquid phase reaction mixture which entail extremely high construction, maintenance and production costs.
Acetic anhydride can be produced by reacting one molecule of the ketene intermediate obtained by the pyrolysis of acetic acid with another molecule of acetic acid. This process has been also obsoleted by the liquid phase carbonylation process of converting methyl acetate with carbon monoxide in the presence of a rhodium catalyst and the methyl iodide promoter, as will be further discussed below.
Methyl acetate, which has been generally prepared by the esterification of acetic acid with methanol, has not been widely used as a chemical raw material due to its high production cost, despite its large potential as a key intermediate to numerous industrially important chemicals including acetic anhydride, ethanol, alkyl acetates, vinyl acetate monomer and the like.
In order to overcome the various problems associated with the liquid phase carbonylation of methanol to produce acetic acid, therefore, various proposals have been made to provide a process for producing acetic acid in a gas phase. For example, the processes disclosed in European Patent Publication No. 0 069 514 A2 assigned to Toyo Engineering Corporation, German DE 33 23 654, and Ind. Chem. Prod., Res. Dev., 22, 436(1983) and Chemistry Letters., 895(1987) relate to the gas phase production of acetic acid by a nickel-catalyzed carbonylation of methanol; however, none of these processes has proven to be commercially viable due to various problems.
European Patent Publication No. 0 335 625 A2 provides a process for producing acetic acid by employing a nickel/ rhodium catalyst supported on active carbon at 188.degree. C. In this process, a mixture of CO/H.sub.2 (1:2) gas is introduced under a pressure of 9 arm, with the ratio of methanol to methyl iodide being 100:19.1 and the LHSV of the feed being 1. However, this process results in a low yield of 9.7%. In addition, nickel is apt to be vaporized from the catalyst beds during the reaction, thereby shortening the life time of the catalyst.
U.S. Pat. Nos. 3,717,670(to Hockman) and 3,689,533 (to Schultz) offer processes for producing acetic acid in a heterogeneous gas phase using a rhodium catalyst. These patents teach that the conversion of methanol and the yield of acetic acid may be improved by mixing the Rh catalyst with a metallic component. However, according to these patents, the methanol conversion, the selectivity and the yield of acetic acid are no more than 78.5%, 58% and 45.5%, respectively, under the most preferred reaction conditions: i.e., a reaction temperature of 285.degree. C. and pressure of 200 psi, with the molar ratio of CH.sub.3 I:CH.sub.3 OH:CO being 1:12.3:26.2.
Japanese Laid-open Patent Publication No. Sho 48-80511 describes a gas phase process for preparing acetic acid wherein a rhodium compound is employed as a catalyst and a small amount of cobalt, nickel or iron salt and/or aluminum, copper, titanium, mercury or lithium salt is added as a co-catalyst. In this method, methanol, carbon monoxide and methyl iodide are introduced at a rate of 169 g/hr, 224 g/hr and 27 g/hr, respectively, using the catalyst prepared by supporting 0.43 g of RhCl.sub.3.4H.sub.2 O, 0.43 g of NiCl.sub.2, 0.44 g of AlCl.sub.3 and 0.43 g of LiCl on 25 g of active carbon; and the reaction is carried out at 230.degree. C. under 220 psi. However, this process gives an acetic acid yield of 71%.
U.S. Pat. No. 4,918,218 to Mueller, et al. and German Patent No. 36 06 169 relate to a gas phase process using a nickel/palladium complex catalyst system and a process using a cobalt catalyst supported on zeolite, respectively. However, neither process has been regarded as commercially viable in view of their low reactivity, conversion and selectivity to acetic acid.
As a separate but related matter, among the various problems that exist in the afore-mentioned gas phase carbonylation processes to produce acetic acid, the most critical impediment to their commercialization has been the short life time of the expensive(e.g., rhodium) catalyst due to its contamination by impurities present in the feed gas, i.e., CO.
As a matter of fact, the task of dealing with the contamination of catalysts is a pervasive one throughout the chemical industry as numerous chemicals are prepared by catalytic reactions using an industrial gas such as synthesis gas(CO/H.sub.2) and carbon monoxide. Representative of such reactions include hydroformylation and carbonylation of various reactants to produce, e.g., acetic acid and acetic anhydride, as discussed above. In these reactions, expensive noble metals, including rhodium, are generally used as a catalyst.
The afore-mentioned industrial gases can be manufactured by various known processes. During the processes, various impurities, especially iron carbonyl compounds, are formed as the gases come in contact with iron. Also, when they are stored in an iron vessel at a room temperature for a substantial period of time, a significant amount of iron carbonyl compounds may be formed.
Said iron carbonyl compounds have been found to cause serious problems in carrying out the above catalytic reactions as they tend to accumulate on the active surface of the catalyst and poison the catalyst rapidly. The iron carbonyl compounds, even in a minor amount, may degrade the catalyst performance, including its reactivity and selectivity, after a repeated use thereof. Accordingly, unless and until a commercially feasible solution is found to remove the catalyst contamination problem, there may be no practicable alternative to, e.g., the existing liquid phase process for the production of acetic acid discussed above.
Turning now to prior art methods of producing acetic anhydride, U.K. Patent No. 1 523 346 teaches a process for preparing acetic anhydride from methyl acetate and carbon monoxide in a liquid phase reaction in the presence of a metallic catalyst such as ruthenium, rhodium, palladium, osmium, iridium and platinum. In accordance with this process, starting materials are preferably used in anhydrous form, but they may contain up to 25% of methanol and 5% of water. In this process, since the water present in the reaction system tends to cause the formation of acetic acid, it is vitally important to remove the water from the reactants in order to obtain acetic anhydride in a higher yield or selectivity.
To solve the problem of removing water encountered in the preparation of acetic anhydride, therefore, European Patent Publication No. 0 087 870 A2 proposes a method comprising the steps of esterifying the produced acetic acid with methanol followed by dehydrating, carbonylating and separating the resulting product. Specifically, this process comprises esterifying methanol with recycled acetic acid to obtain a mixture of methyl acetate, methanol and water and removing the water from the esterification product; further dehydrating the methyl acetate by, e.g., injecting acetic anhydride and carbonylating the dehydrated methyl acetate with CO in a liquid phase to produce simultaneously acetic arthydride and acetic acid depending on the contents of water and methanol in the reactants; separating an overhead fraction containing the carbonylation feed and halide promoter, an intermediate fraction containing acetic acid and acetic arthydride, and a lower fraction containing the carbonylation catalyst components from the reaction mixture; recycling the overhead fraction and the lower fraction to the carbonylation reactor; further separating the intermediate fraction into acetic acid and acetic anhydride; recycling the separated acetic acid to the esterification reactor; and, finally, recovering the acetic anhydride.
Not only is the above process highly complicated and costly, it is very difficult to remove water after the esterification step. For example, when the esterification is carried out using methanol and acetic acid in a molar ratio of 2:1, 57.5% by weight of methyl acetate, 27.9% by weight of methanol and 13.6% by weight of water are produced. The resulting water must be removed by azeotropic distillation of water and methanol. Thereafter, another step is required to separate methanol from water.
In addition to the water removal problem, there exists another cost-related deficiency in the above process for the preparation of acetic anhydride: that is, methyl acetate is prepared by esterifying acetic acid with methanol and the methyl acetate so produced is used as the starting material for the next carbonylation step wherein said acetic anhydride (and acetic acid) is produced, which further increases the overall production cost of acetic anhydride.